Update to rscons 3.1.0

.gitignore

@@ -1,4 +1,2 @@
-/app
-/app.exe
 /build/
 /.rscons*

Rsconscript

@@ -3,8 +3,7 @@ configure do
   check_cfg package: "freetype2"
 end
 
-build do
-  Environment.new do |env|
+env "app" do |env|
   sources = []
   dirs = [
     "src",
@@ -27,6 +26,5 @@ build do
   ]
   env["DFLAGS"] += ["-fversion=Derelict3", "-fversion=gl3n", "-fversion=SDLImage2"]
   env["LDFLAGS"] += ["-static-libgcc"]
-    env.Program("app", sources)
-  end
+  env.Program("^/app", sources)
 end